This invention relates generally to tunable surface acoustic wave technology and pertains more particularly to ZnO based monolithically integrated tunable surface acoustic wave (MITSAW) technology and electronic and photonic systems employing MITSAW devices.
SAW devices have been widely used for signal processing since 1964, when the interdigital transducer (IDT) was introduced. The basic principle of a SAW device is to apply an input IDT and an output IDT in mutually spaced relation to a piezoelectric member, to apply an electrical signal to the input IDT, thereby causing a surface acoustic wave to propagate in the piezoelectric member, and to obtain the electrical signal generated in the output IDT by the propagated surface acoustic wave. The time for the propagated wave to travel from its generation at the input IDT to its arrival at the output IDT constitutes a time delay and the piezoelectric member constitutes a delay path.
A problem in SAW technology to date has been the lack of tunability of acoustic velocity, which would allow tuning of the center frequency of the SAW filters. A conductive element near the piezoelectric surface changes the acoustic velocity by coupling with the electric fields of the acoustic wave. Ideally, tunability of the acoustic velocity is limited by the electromechanical coupling coefficient of the piezoelectric material. Early attempts include the use of a semiconductor film in close proximity to the piezoelectric surface. The variable finite conductivity of the semiconductor interacts with the electric fields associated with the acoustic wave, and slows the wave. An improved approach is to use a two dimension electron system (2DES) to tune the acoustic velocity.
More particularly, the acoustic velocity of the propagated surface acoustic wave is controlled by changing the conductivity of the 2DES through reverse biasing of the quantum well. When the quantum well is depleted, the acoustic wave propagates at the near open-circuit velocity of the piezoelectric layer and its underlying substrate system. On the other hand, when the quantum well is forward biased, the acoustic velocity approaches short-circuit velocity.
In this reported device, a GaAs substrate was used as a piezoelectric medium, and the 2DES was formed in an AlxGa1xe2x88x92xAs quantum well. As the piezoelectric coupling of GaAs is very small, the reported tunability range was  less than 0.1%. An alternative hybrid GaAsxe2x80x94LiNbO3 device where the 2DES was formed in a GaAs quantum well, which was epitaxially lifted off, and bonded to the LiNbO3 substrate. The effective coupling coefficient of this structure was reported to be 3.5% and a velocity tunability of 1.5% was reported. However, the epitaxial liftoff technology is very complicated, with low. yields and poor reliability; therefore, it is unsuitable for commercial applications.
A known SAW filter includes a piezoelectric layer disposed on an underlying substrate, an input IDT and an output IDT A varying electrical signal source applies signals to the input IDT and a load is connected to the output IDT 16.
Modern communications systems are increasingly moving to higher data rates to accommodate the demand for enhanced capabilities, such as data and multimedia communications. Coupled with an expanding user base, this increasing data rate translates to larger bandwidths and higher frequencies in the engineering requirements. High frequency, low loss, low power, miniaturized and integrated filters, for example, are required for the exponentially expanding wireless communications industry. As multi-functional systems are deployed, adaptive and programmable filters are needed.
While digital filter technology is advanced, digital signalprocessors consume high power, are limited to the lower end of the frequency spectrum, being subject to circuit speed limitations and their performance is dependent on the analog preprocessing and post-processing circuits. Analog devices and circuits, which can achieve the aims of high frequency, low loss, low power, miniaturized and adaptive filtering, are needed as low cost, light weight, high performance alternatives.
SAW devices have been widely used in communications systems. They are easy to fabricate and are low cost, light weight, and very versatile devices. High performance filter specifications can be realized, using space-domain sampling, as opposed to time domain sampling in digital techniques. Very complex filter functions can be implemented, with independent design of frequency and phase response.
The major limitation of conventional SAW filters is that their frequency and phase response is set at the time of their design, and cannot be changed during operation. However, in many modem communication systems adaptive signal processing is desired for increased signal to noise performance, and security concerns. Further, tunability of the time or frequency domain response is desirable for the communications system to adapt to its operating environment.
Two distinct types of surface acoustic wave filters have emerged to meet these demands. The first type is the programmable SAW filter, which changes the filter parameters such as center frequency, band width, and pass band shape. This type of SAW device includes filter banks, multiple interdigital transducer (IDT) filters and electrode configurable devices. The second type of SAW filter was created to meet the tunability demands. Early designs were based on the voltage-controlled width of a depletion layer in a semiconductor bulk diode applied, with the input and output IDTs to the piezoelectric member. However, the current programmable SAW filters are large in size, complex and costly with relatively high insertion loss, which render them unsuitable for many applications.
In providing SAW voltage controlled oscillators, the art has typically placed a SAW device in a feedback path, e.g., connecting one SAW IDT to the input terminal of an amplifier and connecting the output terminal of the amplifier to the other SAW IDT.
The frequency tuning range of the voltage controlled oscillator (VCO), which is built on the given piezoelectric material, is inversely proportional to the delay time of the SAW device. The shorter the delay time of the SAW, the higher is the frequency tuning range. Heretofore known SAW VCOs are seen as either having a less than desired operating frequency and frequency tuning range due to small electromechanical coupling coefficients and low acoustic velocity, such as GaAs/AlxGa1xe2x88x92xAs devices, or having complicated structures with low yields and poor reliability due to the hybrid processing technology, such as GaAsxe2x80x94LiNbO3.
Zero-power passive wireless sensors are important for environmental monitoring and identification applications. Their principle advantage is that they do not need a power source, as they derive energy from an interrogation signals. They are particularly attractive for hazardous environments, such as interiors of engines, chemical reaction chambers, high-voltage lines and the like.
Presently there are two known types of zero-power passive SAW wireless sensors. The first type consists of GaAs/AlxGa1xe2x88x92xAs quantum well and SAW structures, which are seen having very small electromechanical coupling coefficients with accompanying low acoustic velocity, resulting in a small dynamic range for wireless sensor read-out. The second type, which consists of GaAs/LiNbO3 hybrid structure, suffers from low yields, poor reliability and high cost due to complicated bonding technology.
The optical delay line technology is used for optical signal processing. Presently known and cost-effective SAW optical delay lines also suffer from the limitation of less than desired electromechanical coupling coefficients. They are further limited in frequency of operation, not operable in the ultraviolet (UV) range.
As TV range lasers are developed, optical signal processing devices operating in this range will be needed, e.g., optical multiplexers, demultiplexers, modulators, delay lines and the like.
Zinc oxide is a versatile semiconductor material, with a wide and direct energy band gap (circa 3.3 eV). It has an exciton binding energy (Eb) of 60 meV, which is 2.4 times the thermal energy at room temperature. The large Eb implies that electron-hole pairs are well bound even at room temperature, and efficient radiative recombination is possible if non-radiative recombination sites caused by crystal defects can be reduced by improving the quality of the film. Recently, ZnO has been used for visible-blind UV photodetectors. Optically pumped laser emission has been observed in ZnO films. This opens up the possibility of developing UV lasers from ZnO films. ZnO based ternary alloys, MgxZn1xe2x88x92xO, have been demonstrated, allowing band gap engineering from 2.8 eV to 4.0 eV. In comparison with other wide band gap emiconductors, ZnO can be grown in the 300 degrees centigrade to 450 degrees centigrade range, hundreds of degrees lower than GaN, xe2x80x9ca cool way to beat bluexe2x80x9d.
ZnO films have recently been used as the substrate or buffer layer for the growth of GaN based optoelectronic devices. The lattice mismatch between GaN and ZnO is relatively small, which makes growth of high quality films possible. ZnO/GaN heterostructures have been used for hybrid opto-electronic devices. GaN films grown on high quality ZnO buffer layers (grown on Cxe2x80x94(Al2O3) have been observed to have better structural properties compared to GaN films grown on sapphire and SiC.
ZnO is well known as a piezoelectric material used in bulk acoustic wave (BAW) and surface acoustic wave (SAW) delay lines, filters and resonators in wireless communication and signal processing. ZnO thin films have been used in conjunction with low loss high acoustic velocity substrates, such as sapphire (Al2O3) and diamond; with semiconductors, such as Si, GaAs and InP; and with low coupling coefficient piezoelectric materials, such as quartz. ZnO thin films deposited on GaAs and on InP are also used for acousto-optic modulators.
The key issue for high performance, thin film ZnO based SAW device fabrication is the control of the film quality. Many growth technologies have been used to grow ZnO films. Among them, MOCVD (metal organic chemical vapor deposition) technology offers the advantages of high quality epitaxial growth on large area substrates in a production scale.
Applicants herein have used an MOCVD system with a rotating disc reactor chamber. ZnO epitaxial films are grown, using DEZn as the zinc precursor and oxygen as the oxidizer. The gas phase reaction between DEZn and oxygen can occur at room temperature and results in particulate formation, which degrades ZnO film properties, including surface morphology and crystallinity. In order to minimize the gas phase reaction, the MOCVD reactor is designed to have a flow-top configuration with high nitrogen push flow. DEZn and oxygen are introduced into the reactor separately. The substrate is rotated at high speed for improving thickness uniformity.
The present invention has as its primary object to provide ZnO based monolithically integrated tunable SAW (MITSAW) devices having a ZnO/MgxZn1xe2x88x92xO quantum well structure.
In attaining the primary and other objects, the invention provides a family of electronic and photonic devices with improved operational characteristics and manufacturability.
The basic MITSAW device of the invention includes a piezoelectric member, input and output IDTs disposed on a surface of the piezoelectric member, and a quantum well structure also disposed on a surface of the piezoelectric member.
The piezoelectric member is comprised of zinc oxide. The quantum well structure is composed of the binary semiconductor of zinc oxide (ZnO) and the ternary semiconductor of magnesium zinc oxide (MgxZn1xe2x88x92xO). A substrate is provided for deposition of the piezoelectric member and is comprised of R-plane sapphire.
The MITSAW device is built using R-plane sapphire substrate instead of the popular C-plane sapphire, which offers unique advantages; (i) the c-axis of the ZnO film in the ZnO/Rxe2x80x94(Al2O3) material system is in-plane, resulting with electrical, piezoelectric and optical anisotropy for novel applications; (ii) certain wave modes in the ZnO/Rxe2x80x94(Al2O3) material system have large coupling coefficients and low loss compared to the GaAs/AlxGa1xe2x88x92xAs material system, which significantly enhances the tunability of the acoustic velocity; and (iii) lattice mismatch between., ZnO and R-plane sapphire is less than that between ZnO and C-plane sapphire, resulting in high quality ZnO thin films.
In a first electronic system aspect, the invention provides a voltage controlled oscillator (VCO) employing a ZnO based MITSAW structure as above described.
In a second electronic system aspect, the invention provides an adaptive and tunable filter employing a ZnO MITSAW structure as above described.
In a third electronic system aspect, the invention provides a zero-power remote wireless sensor employing a ZnO MITSAW structure described above as a tunable delay line which is used as a readout element.
In a fourth system aspect, the invention provides a photonic system having respective fixed and tunable UV optical delay lines using acousto-opto-electronic interaction in the ZnO MITSAW device as above described.
The foregoing and other objects and features of the invention will be further understood from the following detailed description of the preferred embodiments and practices and from the drawings.